"Enkephalin" is the general term referring to Met.sup.5 -enkephalin (A is Met in the below formula) or Leu.sup.5 -enkephalin (A is Leu in the below formula) of the following formula: EQU Try--Gly--Gly--Phe--A
(wherein A represents Met or Leu). These two pentapeptides bind to opioid receptors and such produces analgic effect. They are neurotransmitters (see Nature, 258, 577 (1975)).
Enkephalinase, found by Malfroy et al, is an enzyme which cleaves Met-enkephalin or Leu-enkephalin at the Gly.sup.3 --Phe.sup.4 bond (see Nature, 276, 523 (1978)). It plays an important role in the termination of an analgic effect which enkephalin has.
Accordingly, it is considered that the inhibition for enkephalinase slows down the deactivation of enkephalin and that the analgic effect maintains.